Free Public Reason examines the idea of public justification, stressing its importance but also questioning the coherence of the concept itself. Although public justification is employed in the work of theorists such as John Rawls, Jeremy Waldron, Thomas Nagel, and others, it has received little attention on its own as a philosophical concept.

D'Agostino shows that the ideal behind this concept is constituted by many, sometimes competing, demands and that no formal way of weighing these demands can be identified. The notion of public justification itself is thus shown to be contestable. In demonstrating this, D'Agostino questions many current political theories that rely on this concept.

Having broken down the foundations of public justification, D'Agostino then draws on the ideas of Dworkin and Kuhn as well as insights from feminism and post-modernism to offer an alternative model of how a workable consensus on its meaning might be reached through the interactions of a community of interpreters or delegates at a constitutional convention.